In semiconductor material, light with a wavelength between 300 nm and 1100 nm attenuates generating charge carriers (i.e., holes and electrons). In an intrinsic material, the small concentration of holes equals the concentration of electrons. In contrast, a doped material has a higher concentration of one charge carrier (e.g., more electrons than holes). A doped material that has more electrons is called n-type, while a doped material that has more holes is called p-type. Heavily doped materials, indicated by the addition of a plus sign after the n or p (e.g., n.sup.+), have an overabundance of one carrier.
Semiconductor materials generally have a select number of bound electrons which result from charge attraction with the nucleus. However, semiconductors may also have unbound electrons that form a "sea" of electrons that can move around freely in the material. Some of the unbound electrons may get excited and become free from their current location, or valence band, and to the conduction band. The energy difference between the valence band and the conduction band is defined as the band gap. The band gap between the valence and conductance bands for materials such as silicon is 1.1 eV which corresponds to a wavelength in the near infrared region of the electromagnetic spectrum (approximately 1120 nm).
Once charge carriers are created, they can be used to generate electrical signals. As would be apparent to one of ordinary skill, the movement of electrons generates a current which can be detected by various types of conventional detectors. Thus, light can be detected.
Often conventional avalanche devices may have a p-i-n structure. One of ordinary skill will recognize it as a semiconductor in which the positively doped layer (p) is separated from the negatively doped layer (n) by an intrinsic layer (i). The p-i-n structure can present such disadvantages as high working bias (hundreds of Volts), need for cooling systems, and difficulty in the production of large area and multi-channel elements with high gain, which strongly limits their use to a relatively small number of applications.
Other silicon avalanche detectors with negative feedback are based on CRS (Conductor-Resistor-Semiconductor) structure. Negative feedback, as known by those skilled in the art, is a method by which the process of avalanching can be limited at an appropriate point. FIG. 1 illustrates a conventional CRS avalanche detector with negative feedback. The conductive layer 100 is next to the SiC (resistive layer) 105. The resistive layer 105 is separated from the bulk semiconductor 110 by an SiO.sub.2 (isolating layer) 115. The bulk semiconductor 110 in conventional CRS detectors generally has p-type doping with a lower p.sup.+ layer 120 serving as a second conductive layer.
The CRS structure can supply a feasible alternative, free of many of the disadvantages of devices with p-i-n structures. CRS detectors can be operated at room temperature and relatively low voltages (approximately 40 to 50 V), with a multiplication coefficient as high as 10.sup.4 -10.sup.5, while providing good time resolution (less than 600 ps). They are routinely fabricated on the low-cost, low-resistivity silicon substrates, where both detector and preamplifying electronics can be easily integrated. Although initial results of conventional CRS detector tests are encouraging, especially since they have good spectral efficiency for the long wave range of light spectrum, there are some shortcomings in the CRS design that limit their abilities to detect light.
When the traditional CRS design is used, it is essentially only sensitive to the red portion of visible light. Another drawback is the requirement for the resistive layer to be transparent for the blue light, while having an electrical resistance high enough to quench avalanche multiplication. In addition, present CRS detectors have a diffusion zone behind the sensitive depleted region which can decrease time resolution and increase noise resulting from delayed charge carriers. Another disadvantage of CRS detectors is that a significant amount of the diode area is passive. The passivity which results from the circular geometry of the heavily doped n regions can hinder the enhancement of the electric field necessary for avalanche multiplication.